This invention relates to the selective absorption of a specified gas or gases from a gas stream and in particular to gas purification, especially the removal of some or all of acidic gas components, such as hydrogen sulphide and carbon dioxide, from gas streams such as natural gas and industrial gases by chemisorption.
Hydrogen sulphide and carbon dioxide often occur in gas streams and it is often desirable to reduce the concentration of hydrogen sulphide and/or carbon dioxide, to a very low level since the acid gases may act as poisons for catalysts with which the process gas is to be contacted or may otherwise be objectionable.
Selected gases may be removed from process gas streams by contact of the gas stream with a liquid absorbent composition that absorbs the specified gas or gases. The loaded absorbent is then separated from the process gas and then subjected to a regeneration process to recover the absorbent for recycle to the absorption step. Where it is desired to reduce the concentration of the specified gas or gases to a very low level, the absorbent is generally selected to be one that reacts with the specified gas or gases. Examples of absorbent liquids suitable for the absorption of acid gases include aqueous solutions of alkanolamines, such as monoethanolamine (MEA), diethanolamine (DEA) and methyl diethanolamine (MDEA), alkali carbonates or phosphates, for example potassium carbonate and tripotassium phosphate, and glycol-amine mixtures.
The absorption of the specified gas or gases by such absorbents is favoured by low temperatures and high pressures, and vice-versa for desorption, i.e. regeneration. Accordingly the absorption step is normally effected by contacting the process gas at a relatively low temperature but at an elevated pressure in an absorption column with a stream of the absorbent liquid flowing counter-current to the process gas. The specified gas or gases are absorbed by the absorbent giving a loaded liquid absorbent stream and a treated process gas stream containing a reduced proportion of the specified gas or gases. For convenience such a treated process stream containing a reduced proportion of the specified gas or gases is herein termed a sweet process gas. The loaded absorbent is separated from the process gas and then regenerated. The regeneration may involve reducing the pressure and/or heating the loaded absorbent. The specified gas or gases are often stripped from the loaded absorbent by feeding the loaded absorbent to the upper end of a regeneration column provided with a reboiler at the lower end to vaporise part of the absorbent. The vapour, which is primarily water vapour in the case of the aforementioned aqueous solutions since the solute generally has a relatively low volatility, passes up through the column in counter-current vapour/liquid contact with the loaded absorbent: as a result the specified gas or gases are is stripped from the loaded absorbent liquid. At the top of the column, the vapour is condensed and returned to the column. At the lower end of the column, a stream of regenerated, i.e. "lean", absorbent liquid is taken and recycled to the absorption stage. Usually heat exchange is effected between the cold, loaded, absorbent liquid stream taken from the absorption stage and the hot, lean, absorbent stream taken from the stripping column prior to return of the latter, generally after further cooling, to the absorption stage. The most severe disadvantage of this arrangement is that it cannot reduce the concentration of the specified gas or gases in the sweet process gas to a value lower than a few ppm by volume without using excessive amounts of energy. Because of this it is common practice to require a further purification process, for example based upon adsorption technology, to operate on the sweet process gas downstream of the absorption plant when it is required that the concentration of the specified gas or gases is reduced to a very low level.
In order to improve the efficiency of the above "single-loop" process, it is known to employ a split stream, i.e. a "double-loop", arrangement. In the double-loop process, the regeneration is effected to give a partially regenerated ("semi-lean") absorbent. Part of this semi-lean absorbent is subjected to further regeneration to give the lean absorbent liquid which is returned to the absorption stage at a location adjacent the outlet of the sweet process gas. The remainder of the partially regenerated, i.e. semi-lean, absorbent is returned to the absorption stage at an intermediate location.
However the double-loop systems have several drawbacks. Firstly this process also has high energy requirements when removing the specified gas or gases to very low concentrations. Secondly, the process uses higher absorbent flow rates than a single-loop process, and thirdly there are practical problems associated with controllability.